TECHNETIUM-99M GENERATOR By: NOR SYAKIRAH BINTI ABD
MUTALIB
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INTRODUCTION Technetium-99m (chemical symbol Tc-99m) is a
metastable form of radioisotope Tc-99 widely used in nuclear
medicine. Technetium-99m when used as a radioactive tracer can be
detected in the body by medical equipment such as gamma cameras. It
is well suited to the role because it emits readily detectable 140
keV gamma rays. It is a decay product of its parent nuclide Mo-99
and has a half-life of 6.02 hours. The short physical half-life of
the isotope and its biological half-life of 1 day (in terms of
human activity and metabolism) allows for scanning procedures which
collect data rapidly but keep total patient radiation exposure low.
The same characteristics make the isotope suitable only for
diagnostic but never therapeutic use.
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Technetium-99m was discovered in 1937 as a product of cyclotron
bombardment of molybdenum. This procedure produced molybdenum-99, a
radionuclide with a longer half-life (2.75 days), which decays to
Tc-99m. At present, molybdenum-99 (Mo-99) is used commercially as
the easily transportable source of medically used Tc-99m.
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TC-99M GENERATOR Tc-99m's short half-life of 6 hours makes
storage impossible and would make transport very expensive. Instead
its parent nuclide Mo-99 is supplied to hospitals after its
extraction from the neutron-irradiated uranium targets. It is
shipped by specialised radiopharmaceutical companies in the form of
Tc-99m generators. The generators, colloquially known as a moly
cows, are devices designed to provide radiation shielding for
transport and to minimize the extraction work done at the medical
facility. These generators' output declines with time and must be
replaced weekly, since the half-life of Mo-99 is still only 66
hours.
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Plot of typical Mo-99 and Tc-99m activity on a logarithmic
scale versus time for multiple elution of a technetium generator.
Source: The National Academies Press
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TYPES OF GENERATORS Column generator (most widely used
commercially) - Dry type - Wet type Solvent generator Heat
generator
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Wet-type generator Dry-type generator
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COMPONENTS Column, usually glass, contains a bed of aluminium
oxide (alumina) as a support for the parent radionuclide. Tubing
allows the column to be washed with a sterile saline solution.
Filters will be found in generators in the form of porous frits,
which serve to contain the alumina within the column, and usually a
0.22 m filter which serves to remove any small particles from the
eluted sample and to act as a safety device to ensure a sterile
product. Lead shielding is required for operator safety. All
generators will provide a lead shield around the column and outlet
tubing. Generators are contained in a plastic housing and have some
method (handles, straps) to allow manual or mechanical lifting and
positioning. Source: IAEA Human Health Campus
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ELUTION PROCESS The process of milking Tc-99m is called
elution. Mo-99 in the form of molybdate, MoO 4 2- is adsorbed onto
acid alumina (Al 2 O 3 ). When the Mo-99 decays it forms
pertechnetate TcO 4 -, which, because of its single charge, is less
tightly bound to the alumina. Pouring 10mL of isotonic saline
solution through the column of Mo-99 elutes the soluble Tc-99m,
resulting in a saline solution containing the Tc-99m as the
pertechnetate, with sodium as the counterbalancing cation. The
solution of sodium pertechnetate may then be added in an
appropriate concentration to the organ-specific pharmaceutical to
be used. A large percentage of the Tc-99m generated by a generator
is produced in the first 3 parent half-lives, or approximately one
week.
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Each eluate of the generator should not contain more than
0.0056 MBq, 0.15 Ci of Mo-99 per 37 MBq, 1 mCi of Tc-99m per
administered dose at the time of administration, and not more than
10 g of aluminum per mL of the generator eluate, both of which must
be determined by the user before administration.
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An elution vial
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PRECAUTIONS AND REMINDERS Tc-99m generators received in advance
of the calibration date and time will contain higher amounts of
radioactive material. Care should be taken to assure that the
generator is properly shielded. As in the use of any radioactive
material, care should be taken to minimize radiation exposure to
the patient and to ensure minimum radiation exposure to
occupational workers. Since the eluate does not contain an
antimicrobial agent, it should not be used after 12 hours from the
time of generator elution. The generator should not be used after
16 days from the date and time of calibration. At time of
administration, the solution should be crystal clear.
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DISPOSAL OF GENERATORS Users should monitor the amount of
radioactivity present prior to disposal of the unit. Storage and
/or disposal of the Tc-99m generator should be in accordance with
the conditions of Agreement State or Licensing State licenses and
regulations, or other regulatory agency authorized to license the
use of radionuclides. Vials and needles used for eluting may be
discarded after two (2) days. Tc-99m generators should not be
discarded in ordinary trash within 70 days of the calibration date.
Tc-99m generators of 153.2 GBq, 4140 mCi may be returned to the
manufacturer, while those of 183.9-614.2 GBq, 4970-16600 mCi must
be returned to the manufacturer.
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REFERENCES Amersham Healthcare. 1997. Technetium Tc 99m
Generator For the Production of Sodium Pertechnetate Tc 99m
Injection. Illinois. IAEA Human Health Campus. Design principles of
the 99Mo 99mTc radionuclide generator.
http://nucleus.iaea.org/HHW/Radiopharmacy/VirRad/Eluting_the_Gen
erator/Generator_Module/Design_principles/index.html [2 July 2014].
The National Academies Press. 2009. Medical Isotope Production
Without Highly Enriched Uranium. Washington, DC.